An automotive power window system, proposed in JP-A-2002-168049 or U.S. Pat. No. 5,734,245 (JP-A-8-165842), has a function of preventing an object from being caught between a window glass and a window frame. In such a system, a window closing speed is reduced from a normal speed to a low speed when the window glass reaches a position adjacent to a closed position. As a result, only a small load is applied to the object in the event that the object is caught between the window glass and the window frame.
To detect an object that is an obstruction to window closing operation, a variation in a rotation speed of a motor is calculated when the rotation speed is reduced. Presence of an obstruction is determined when the variation is larger than a predetermined threshold. When an obstruction is present, the rotation speed is reduced and a large variation in the rotation speed is produced. Namely, the presence of the obstruction can be determined by comparing the variation with the threshold.
The variation transitorily becomes large immediately after the rotation speed of the motor is reduced from the normal speed to the low speed. Then, the variation becomes small and the rotation speed becomes stable after a while. The transitory large variation in the rotation speed looks as if the variation is caused by the obstruction. Thus, erroneous determination of presence of an obstruction may be made.
To solve this problem, the obstruction determination is halted for a predetermined period until the rotation speed becomes stable after the reduction of the rotation speed. However, an obstruction can not be detected during this period even it is present. If the obstruction is not successfully detected, it may not be released within a short time.